Overall length is a factor as well as wheel base. If you stretch the rear axle placement out to the far (rear) end of the frame, you might want to consider a 16K ft axle, as the weight will be forward biased.
My rig is 42' twin screw & has a 16k ft axle. This may be overkill however, that is how most of the "big boys" in Elkhart spec them out & I figured that maybe they knew what they were doing.
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